1. Field of the Invention
The present invention relates to fuels, particularly aviation gasoline (Avgas) formulations, which contain reduced amounts of tetraethyl lead. More specifically, the present invention relates to an aviation gasoline formulation possessing a high motor octane number which contains reduced amounts of tetraethyl lead and a method of economically making the aviation gasoline formulation utilizing available excess methyl tertiary butyl ether (MTBE) plant production capacity.
2. Brief Description of Art
Aviation gasoline (Avgas) generally contains an aviation alkylate base fuel and a lead-based additive package. A conventional Avgas formulation contains light alkylate, toluene, C4 to C5 paraffins and tetraethyl lead. Current formulations generally comprise 75-92 vol % light alkylate, 5-18 vol % toluene, 3-20 vol % C4 to C5 paraffins and 2-4 ml/gallon tetraethyl lead (TEL). The industry standard Grade 100 aviation gasoline contains up to 4 ml of TEL/gallon of fuel while Grade 100LL (low lead) aviation gasoline contains up to 2 ml TEL/gallon of fuel. Tetraethyl lead is conventionally added as an octane booster to improve the anti-knock properties of the Avgas fuel over the anti-knock properties of the aviation alkylate base fuel.
The use of tetraethyl lead in fuels, particularly in automotive gasolines, has been restricted for many years due, in part, to health and environmental concerns as well as catalyst poisoning effects in automobile catalytic convertors. Aviation gasolines have been allowed to contain tetraethyl lead since no suitable substitute has been found with adequate knock resistance to allow the current fleet of aircraft engines to operate properly. Current U.S. regulations set a maximum amount of tetraethyl lead in aviation fuels at 4.0 ml/gallon. The continued use of tetraethyl lead nonetheless remains an environmental and health concern which has not been completely resolved. The possibility of further restrictions, or a prohibition, on the use of tetraethyl lead in aviation gasolines therefore exists.
Alternatives to the use of tetraethyl lead are known in the fuel art. For example, methylcyclopentadienyl manganese tricarbonyl (MMT) has been used as an antiknock agent in motor fuels since around 1975, first as a supplement to leaded agents, and then as a replacement to produce lead-free gasoline. However, questions have also been raised concerning the production of undesirable emissions using MMT.
The primary oxygen-containing compounds employed in gasoline fuels today are methyl tertiary butyl ether (MTBE) and ethanol. The use of MTBE as an oxygenate in fuels, however, is also currently under investigation due to health and environmental concerns. For example, MTBE has been observed in drinking water reservoirs, and in a few instances, in ground water in certain areas of California and other States. As a consequence, the benefits of having an ether oxygenate such as MTBE in gasolines, particularly motor gasolines, where its use may also create health and environmental risks, is being questioned.
Current legislation restricts the use of MTBE in fuels and its storage in underground tanks. In California, for example, Executive Order D-5-99 and Senate Bill 989 require the use of MTBE in gasoline to be phased-out beginning Dec. 31, 2002. Current proposals would require a prohibition on MTBE in California gasoline containing 0.3 vol % or more of MTBE beginning Dec. 31, 2002. Starting Dec. 31, 2003, the prohibition would be reduced to 0.15 vol % or more of MTBE. A permanent prohibition of 0.05 vol % or more of MTBE in gasoline would begin Dec. 31, 2004.
As a result of restrictions on the use of MTBE, and the phasing-out of its use in California, a concern has been raised over the possibility of stranded investments in MTBE plant production capacity. A need therefore exists to economically utilize this spare capacity. One possible option is to convert MTBE production capacity to the production of di-isobutylene, which is then hydrogenated to form a mixture of isoparaffins, predominately 2,2,4-trimethylpentane or xe2x80x9ciso-octane.xe2x80x9d Iso-octane derived from such a process may then be used to form the aviation gasoline composition of the present invention.
U.S. Pat. No. 5,470,358 (Gaughan) discloses an unleaded aviation gasoline for use in piston driven aircraft having a motor octane number of at least about 98 which comprises an unleaded aviation gasoline base fuel and at least one aromatic amine. The fuel compositions typically comprise from 4-20 wt. % of the aromatic amines and may further contain other conventional octane boosters, such as benzene, toluene, xylene, MTBE and MMT.
U.S. Pat. No. 4,812,146 (Jessup) discloses a liquid fuel composition having an octane rating of at least about 100 comprising toluene, an alkylate and at least two further components selected from the group consisting of isopentane, n-butane and MTBE.
U.S. Pat. No. 5,851,241 (Studzinski et al) discloses an Avgas composition containing a combination of non-lead additives including an alkyl tertiary butyl ether and an aromatic amine as an additive package.
The disclosures of each of the above-referenced patents are incorporated herein by reference.
In view of the current limitations placed on the use of tetraethyl lead and alkyl tertiary butyl ethers such as MTBE, it is desirable to produce Avgas compositions which contain reduced levels of lead, or do not require the presence of lead-based additives or ether compounds, such as alkyl tertiary butyl ethers.
It is therefore an object of the present invention to provide an aviation gasoline formulation possessing a high motor octane number which contains reduced amounts of tetraethyl lead. It is a further object to provide the aviation gasoline formulation without the required addition of octane boosters such as, for example, MTBE or other ether compounds, amine compounds and MMT.
It is another object of the present invention to provide a method of preparing an aviation gasoline formulation possessing a high motor octane number which contains reduced amounts of tetraethyl lead by blending of the aviation gasoline formulation components.
Yet another object is to provide a method of economically making an aviation gasoline formulation possessing a high motor octane number which contains reduced amounts of tetraethyl lead utilizing available methyl tertiary butyl ether (MTBE) plant production capacity.
These and other objects of the present invention will become apparent upon a review of the following description, the FIGURE of the Drawing, and the claims appended hereto.
In accordance with the foregoing objectives, the present invention provides an aviation gasoline (Avgas) formulation possessing a high motor octane number which contains reduced amounts of tetraethyl lead.
The Avgas formulation of the present invention preferably comprises about 20 to about 80, more preferably about 30 to about 70, and most preferably about 40 to about 60 vol % iso-octane, about 5 to about 18 vol % toluene, about 1 to about 20 vol % C4 to C5 paraffins, from 0 to about 1 ml/gallon tetraethyl lead (TEL) and the balance comprising light alkylate.
In a related embodiment, the aviation gasoline formulation is produced by blending of the formulation components or as a product of refinery operations. More particularly, the Avgas composition of the present invention may be produced utilizing spare or excess MTBE plant production capacity.
In an additional embodiment, the present invention provides a method for operating an aircraft having a spark-ignited internal combustion engine, comprising introducing the aviation fuel composition of the present invention into the engine, and combusting the aviation gasoline in the engine.
In a further embodiment, the present invention also provides a method for preparing a reduced lead content aviation gasoline while maintaining a high motor octane number by blending an aviation gasoline with iso-octane and, optionally, toluene.